Rotors and shafts may operate in a supercritical regime (i.e., at rotational frequencies above the resonant frequency of the shaft and any supporting bearings). Higher speeds may enable higher energy and power density to permit using lighter weight components. Practical applications may include, by way of example and not by way of limitation, flywheel energy storage, airplane engines, and helicopter drive shafts. As long as a rotor is rigid, supercritical operation generally gives stable performance. As the rotational frequency increases, a rotor may eventually experience flex-mode or bending-mode vibrations. Passing through the resonant rotational frequency of such vibrational modes may be problematic. Energy may be dissipated in a resonant frequency rotational mode. Power from a drive source may need to be greater than the energy dissipated in order to continue driving the rotor or shaft. During times when greater energy may be applied from a drive source, considerable stresses may be exerted on the rotor or shaft. Vibrational amplitude when greater energy may be applied from a drive source may be so large that the rotor may impact surrounding structure about the rotor or shaft.
Previous techniques for damping shaft vibrations may have included squeeze-film dampers, active magnetic bearings that apply control forces to the rotating shaft or linear actuators that contact conventional bearing housings and apply control forces to the bearing housings. Piezoelectric materials may have been used in structures arranged to damp vibrations in beams. Piezoelectric materials may have been used as an analog to magnetic bearings in which an applied voltage may control a piezoelectric patch to apply a force to directly oppose motion of a beam to which the piezoelectric patch may be mounted. Variable stiffness to control vibration may have also included shape-memory alloys and magnetorheological elastomers.
Variable stiffness properties of piezoelectrics may have been used to semi-actively tune vibration absorbers, such state-switched absorbers may change the resonant frequency of a beam and may be matched to optimally transform vibrational energy into electrical energy.
There is a need for an apparatus and method for affecting physical parameters of a shaft including changing frequency of vibrational modes of a shaft to reduce vibration while rotating the shaft, especially at resonant frequencies.